1. Field of the Invention:
The present invention relates to a process and an apparatus for the preparation of synthesis gas. The preparation process includes catalytic steam and/or carbon dioxide reforming of a hydrocarbon feedstock. In particular, the invention provides an improved process of the above type including the steps of heated steam reforming of a hydrocarbon steam mixture in contact with catalysed hardware having activity in steam reforming and subsequently reforming the partially reformed effluent in a fired steam reformer.
2. Description of the Related Art:
Partial steam reforming upstream of a fired steam reformer in the form of pre-reforming of a hydrocarbon feedstock in the preparation of synthesis gas is well known in the art. Pre-reforming is generally employed with hydrocarbon feed containing higher hydrocarbons or for increasing the capacity of the existing reformer plants. Process gas of the hydrocarbon feedstock and steam and/or CO2 is thereby introduced in a pre-reformer at temperatures of about 450° C. to 550° C. By the steam reforming reactions proceeding in the pre-reformer, the temperature in the process gas usually decreases or increases slightly when carrying out the pre-reforming process depending on the hydrocarbon feedstock as it is an adiabatic operation.
In industrial synthesis gas preparation plants, the pre-reformed process gas to which CO2 may be added is subsequently reheated to the desired inlet temperature to the fired steam reformer by heat exchange with hot flue gas from the fired steam reformer. The usual inlet temperature into an industrial reformer is between 600° C. and 700° C.
Introducing a flue gas heated steam reforming step in between the pre-reformer and the fired steam reformer will result in an increased utilisation of the flue gas heat content, while it is possible to keep the inlet temperature between 600° C. and 700° C. However, the application of the process of the invention is not limited to this temperature interval.
Increased utilisation of the heat content in the flue gas for reforming is desirable as it reduces the size of the fired reformer and reduces the waste heat used for steam generating thereby limiting the steam export, which may be undesirable.
Improved utilisation of heat in the hot flue gas from the fired steam reformer is disclosed in EP patent application No. 855,366. This publication describes a process whereby process gas to the steam reformer is partly reformed in a pre-heater coil provided with a thin film of steam reforming catalyst on the wall of the coil. A high amount of valuable heat in the flue gas is then transferred to and absorbed by the process gas through endothermic steam reforming reactions proceeding on the wall-coated catalyst. The coil dimension and amount of catalyst is thereby adjusted to increase the exit temperature in the partially reformed process gas from the catalysed pre-heater coil to the required temperature at inlet to the fired steam reformer.
The main disadvantage of this process is decreasing catalyst activity at long time operation of the catalysed preheater coil. This results in a coil exit temperature above the maximum allowable gas temperature at the inlet of the fired steam reformer. The increased coil exit temperature is due to decreased heat absorption at diminished steam reforming in the gas. The catalyst has then to be reactivated or replaced with fresh catalyst on the coil wall. Replacement of catalyst in the pre-heater coil is a difficult and expensive operation when demounting the coil from the flue gas channel.
The objective disclosed in EP patent application No. 1,069,070, which is incorporated herein by reference is to improve long term operability of a steam reforming process of the above type by compensating a decreasing catalyst activity of the thin film catalyst applied to the wall of the pre-heater coil by means of an additional catalytic unit being easy to replace.
This publication discloses a process for the catalytic steam reforming of a hydrocarbon feedstock and includes steam reforming a hydrocarbon steam mixture in contact with a first steam reforming catalyst being arranged as a thin film on the wall of the catalysed pre-heater coil in a flue gas channel from a fired steam reformer. Contacting partially reformed effluent from the catalysed pre-heater coil with a second steam reforming catalyst in a fired steam reformer follows this step. The process includes the further step of contacting the partially reformed effluent with an intermediate reforming unit arranged between the outlet of the catalysed pre-heater coil in the flue gas channel and the inlet of the fired steam reformer.
Loss of activity in the catalysed pre-heater coil unit during long time operation is partially compensated for by steam reforming reactions in partially reformed effluent within the intermediate reforming unit. The intermediate unit is operated at substantially adiabatic conditions and compensates partially decreasing steam reforming activity of the thin film steam reforming catalyst on the catalysed pre-heater coil, and the resulting temperature increase in the effluent from the catalysed pre-heater coil.
Besides providing the required temperature adjustment of the process gas below the maximum inlet temperature into the fired steam reformer at long term operation, a further advantage of the intermediate reformer unit is the siting of the unit outside the flue gas channel. To compensate decreasing activity in the catalysed pre-heater coil as described above, it will be necessary to replace or reactivate spent catalyst upstream the fired steam reformer. As mentioned earlier replacement of spent catalyst applied as a thin film to a coil within the flue gas channel is time consuming and expensive to handle.
By arranging an intermediate catalyst unit outside the flue gas channel, spent catalyst is then replaced in the intermediate reformer unit and the replacement operation is simplified considerably.
In a system where the catalysed pre-heater coil is designed such that the process gas leaving the coil is in chemical equilibrium at the desired outlet temperature, the intermediate reforming unit, when operated adiabatically, will not change the temperature or the gas composition. As the catalyst in the catalysed pre-heater coil deactivates the chemical reactions will not be in equilibrium. This means that less heat is used for carrying out the endothermic steam reforming reaction and given a virtually unchanged amount of heat transferred to the catalysed pre-heater coil, more heat is available for heating. This results in an increased outlet temperature from the coil. In this case the intermediate reforming unit will bring the gas composition closer to equilibrium, thereby cooling the gas to a temperature close to the desired temperature achieved before deactivation of the catalyst in the catalysed pre-heater coil.
However, as the deactivation of the catalyst in the catalysed pre-heater coil becomes severe the resulting temperature increase becomes a problem. The temperature of the pre-heater coil increases, which may exceed design temperatures, resulting in a smaller driving force for heat transfer from the flue gas leading to a smaller transferred duty, with the consequence that the capacity of the total reforming system has decreased. The use of the intermediate reforming unit does not solve these issues, and replacement of the thin film catalyst applied to the wall of the pre-heater coil becomes necessary.
The processes described in EP 855,366 and EP 1,069,070 both have the disadvantage of difficult replacement of the thin film catalyst on the wall of the catalysed pre-heater coil in the flue gas channel. EP 1,069,070 describes a partial solution, which prolongs the useful life of the thin film catalyst on the wall of the reheat coil in the waste heat section. However, deactivation of the thin film catalyst on the wall of the reheat coil in the waste heat section is expected with time to eventually necessitate replacement of this catalyst. As explained above this operation undesirable as it is time consuming and expensive.
U.S. Pat. No. 3,743,488 describes a process in which the hydrocarbon steam mixture is repeatedly heated in a flue gas stream and reacted in adiabatic reactors external to the flue gas stream, with steam reforming catalyst pellets. This concept offers easier access for change of the catalyst in the external reactors. However, the use of many adiabatic reactor vessels is overall an expensive solution.
The process described in U.S. Pat. No. 4,959,079 is designed with the purpose of improved utilisation of heat in the hot flue gas from the fired steam reformer. In the process the process gas to the steam reformer is partly reformed in a pre-heated section of the reformer tube that extends from the radiant chamber. Valuable heat in the flue gas is then transferred to and absorbed by the process gas through endothermic steam reforming reactions. However, heat exchange in counter current flow between the flue gas and the reforming tube is poor. Introducing fins on the reforming tube increases the heat transfer. Despite this the amount of heat transfer possible is relatively limited if the reformer tube length is to be kept at a reasonable length.